<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <title>菜鸟教程(runoob.com)</title>
  <style>
    #main {
      display: flex;
      width: 220px;

      /* height: 300px; */

      /* border: 1px solid black; */
    }

    #main div {
      flex: 1;
    }
  </style>
</head>

<body>

  <div id="main">
    <div style="background-color: coral;">红色</div>
    <div style="background-color: lightblue;">蓝色</div>
    <div style="background-color: lightgreen;">带有更多内容的绿色 div</div>
  </div>

</body>

</html>